Note: Descriptions are shown in the official language in which they were submitted.
W O 94/26488 PCT/SE94/00287
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Method and arrangement for pre-pressing fiber material at
board manufacture
This invention relates to a method and an arrangement for pre-
-pressing a mat formed of disintegrated . lignocellulose-
containing fiber material prior to its final pressing at the
continuous manufacture of board, such as fiberboard and particle
board.
Fiberboard normally is manufactured in the form of MDF (Medium
Density Fiberboard), which is a board product based on wood and
which in recent years has been used to a rapidly increasing
extent. MDF is to be understood here as fiberboard with varying
density manufactured according to the dry method. MDF is made
of wood fibers, which in known manner are dried, glued, formed
and pressed in a hot press. Almost without exception the fiber
web or mat formed is a so-called single-layer board, i.e. it
has a substantially homogenous structure with uniform fiber
distribution, uniform moisture content and uniform glue addition
across the thickness. The relatively dry surface layer of fluff
fibers is hereby exposed to radiation heat and contact heat in
the hot inlet to the hot press, which normally is a continuous
press.
Particle board is manufactured in a similar way and is now mostly
built up as so-called triple-layer board, i.e. it comprises a
central layer of coarse chips and two surface layers of fine
chips. These layers are manufactured separately and,therefore,
it is also possible to select different moisture content and
glue content in the layers. At the manufacture of particle board
the surface layer is also exposed to the press heat, but is not
dried out with the same intensity, because of the higher moisture
content in the surface layers and of the more compact chip
material.
At the manufacture of board of the said and simil2r kind, as
mentioned, a web or mat is formed which is pre-pressed and possibly
pre-heated prior to its feed to the hot press where the pressing
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is carried out at controlled surface pressure and/or thickness
at a temperature of 15o-23ooct and where conventional urea
formaldehyde glues are used. (Other glues also are used, espec-
ially at high board densities). In order to bring about necess-
ary board properties, a continuous press is required which is
flexible, and at which a.o. a high surface pressure can be
applied at an early stage in the press. This implies at the
same time, that the thickness of the mat already at this early
stage is very close to the final pressed thickness, i.e. the
mat thickness must be reduced very substantially in the press
inlet.
In order to ensure that such a reduction in thickness takes
place without destraying or attenuating the surface layers of
the mat, the inlet portion must be long and preferably wedge-
-shaped, as this provides the time required for air enclosed
in the mat to be transported out of the mat in a gentle way.
Such an inlet, however, causes the surface layer to be heated
substantially and dried-out in a position where the surface
pressures required for compressing the mat still
are very low. At this method, therefore, the surface particles
are dried out, whereby also the glue dries out and is inactiv-
ated, which results in an unsatisfactory hardness and strength
of the surface layer. The surface layer obtained as a result
thereof often is called pre-hardening layer, because the glue
there has hardened and/or been dried out before sufficient
surface pressures bringing about good contact between fibers
or particles have arisen. This surface layer must be ground in
a later production step and, thus, constitutes a substantial
loss of raw material and handling.
It should also be mentioned in this connection that the press
temperature at the beginning of the press cycle, i.e. in the
inlet portion (compression portion) of the press should be
as high as possible in order to soften the surface layer as
rapidly as possible when surface pressure has been applied,
and in order to obtain the highest possible heat penetration
rate in the board. This desire, thus, is in direct conflict with
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the problem complex of pre-hardening.
Another factor promoting the increase of pre-hardening is the
fact that known continuous presses are provided with endless
conveying belts of steel, and that these belts require large
radii of curvature, of the magnitude 800-1000 mm, which renders
heating in the inlet unavoidable.
When instead pre-hardening is to be minimized, the aim must be
to compress the mat as quickly as possible to sufficient
surface pressure for good bonding. With known designs of
continuous presses it is possible to reduce pre-hardening
to some extent, but there is instead the risk of attenuations
and surface cracks arisiny in the surface material, because
enclosed air, which must be pressed out rapidly, causes over-
pressure in the mat. Such faults may be discovered only at
a much later date. Such surface cracks, for example, often
can pass unnoticed through the board production, and first when
the board is to be painted at the customer the surface cracks
are found to have caused variations in the surface density,
resulting in varying paint suction and thereby varying glaze.
This gives without fail rise to complaints.
The method and arrangement according to the invention solve the
aforesaid problems and simultaneously yield additional advant-
ages. The said pre-hardening layer, for example, can be minimized
or prevented and the evacuation of air takes place more
gently.
The characterizing features of the invention are defined in
the attached claims.
The invention, thus, implies that a successive re-compression
of the fiber mat compressed and expanded in connection with
the forming is carried out without the addition of heat. There-
after the mat is introduced as far as practically possible
into the inlet portion of the hot press where the,mat is trans-
ferred to the hot surfaces in the hot press, whereby high surface
pressures can be applied immediately at a minimum pre-hardening
of the surface layers.
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The invention is described in greater detail in the following,
with reference tn the accompanying drawings showing an embodiment
of the invention.
Fig. 1 is a lateral view of an arrangement according to the r
invent.ion,
Fig. 2 is an enlarged section of the feed area in the hot press
according to a conventional design,
Fig. 3 is an enlarged section of the feed area in the hot press
comprising an arrangement according to the invention.
The inlet portion of the continuous hot press shown in the Figures
is designed in known manner with front guide rollers 1 and heating
plates 2, which are formed with an inlet radius of the same magn-
itude as the radius of the guide rollers 1, and thereafter trans-
form to a substantially parallel portion 4. The distance between
the heating plates 2 can only to a small extent be varied in
relation to the distance at the inlet radius. A steel belt 3
is stretched over guide rollers and drive rollers and slides or
rolls in known manner to the heating plates.-The transition
between the inlet portion and parallel portion 4 is marked by the
centre line 5.
In said inlet portion the pre-pressing arrangement according to
the invention is located. This arrangement comprises three main
parts: a converging inlet and compression portion 7, one or several
nip roll pairs 8, and a slightly diverging delivery portion 9.
The inlet opening 10 of the inlet portion is adjustatle in some
suitable way, automatically or manually, to be adapted to the
height of the incoming mat 11. Hereby, in combination with a
suitable length of the inlet portion 7, the pressing of air out
of the mat 11 takes place in a gentle way without risk of damages.
To the upper nip roll 8 movable in vertical direction a suitable
load is applied so that the desired compression of the mat is
effected. It is expedient here to compress to a density close to
and preferably immediately below the density achieved at the previous
compression after the forming operation. Such a re-compression
requires a relatively moderate load. The load is applied prefer-
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ably by air cylinders, hydraulic cylinders or the like. The end
of the inlet portion 7 closest to the upper nip roll 8 preferably
is coupled together mechanically with the nip roll so as to follow
the vertical movement of the nip roll.
In the subsequent diverging delivery portion 9 the mat expands
slightly, of the magnitude 5-15%, which reduces substantially the
power required for holding the mat compressed.Hereby this portion
can be dimensioned moderately. The ends of the delivery portion 9
closest to the nip rolls 8 also are coupled together mechanically
with their respective nip roll.
The mat 11 is transported through the arrangement between two end-
less belts 12, which can be solid, air permeable or formed as wires.
In the inlet portion 7 the belts are supported on rolls and/or
sliding surfaces. At the outlet end of the delivery portion 9
the belts are broken over a small radius 13 formed as sliding
nose or rollers.
The belts 12 are driven and guided in known manner. If deemed
sui~table in view of forces acting on the belts in the inlet portion
7 and nip 8, stronger inner belts can be used.
EXAMPLE
Conventional design (Fig. 2): At the manufacture of net 19 mm board
a normally pre-pressed fiber mat for the hot press is assumed to
be about 160 mm thick. Without the propused invention, the mat
there is compressed from 160 mm to about 25 mm in the inlet nip
of the hot press. The surface pressure at A is assumed having in-
creased to a level, at which good bonds between fibers and particles
can be obtained at the hardening of the glue. The thickness here
is assumed to be about 50% greater than in the inlet nip. The
distance from the mat contact to A is designated by a.
Design according to the invention (Fig. 3): In this case the mat
has been re-compressed to high density in the inlet and expands
slightly before being passed into the inlet of the hot press. The
distance from mat contact to A is Fig. 3 is designated by b. At the
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entrance into the inlet of the hot press the thickness of the mat
preferably is 1,3-2 times the thickness of the finished board.
At normal designs of the inlet portion in the hot press, the
distance b can be reduced by the invention to a fraction Pf a.
Typical values are of the magnitude 10-30%. The pre-hardening
decreases thereby approximately correspondingly. It is hereby
also possible to increase the steel belt temperature. Additionally,
the mat is not exposed to radiation heat from the hot upper belt
before the mat contact, whereby pre-hardening also is reduced.
The angle at which the steel belt meets the belt is a measure
of the compression rate of the mat. It is understood from the
Figs. 2 and 3 that the angle c~ is more than twice the size of
the angle B, which should be smaller than 15, preferably smaller
than 10. It is also easily understood that the amount of air
to be evacuated is 3-4 times greater in Fig. 2 compared with
Fig. 3. The result of all this is that the risk of surface cracks
and attenuation has been reduced substantially by the invention
in Fig. 3.
At the manufacture of board with normal thicknesses in continuous
presses, for example at thickness 19 mm, often thickness tolerances
after the finishing press of about +/- 0,15 mm are obtained,
which normally is sufficient for selling the board unground.
Owing to the pre-hardening of the surfaces, however, the pressing
must take place at a nominal thickness of about 20,2 mm. This
excess in size, therefore, must be reduced by grinding, i.e.
the result is a direct production loss of raw material (wood
raw material, glue, wax), drying energy, grinding energy, of
6-7% plus the total cost of the grinding operation. Elimination
of pre-hardening, thus, implies an essential saving, which pays
for the investment of the invention in a short time.
Another essential advantage of the invention is that the surface
layers are hard and less paint sucking while maintaining a bright
surface. (In contrast to when water is sprayed through nozzles
on the surfaces prior to pressing).
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The invention, of course, is not restricted to the embodiment
shown, but can be varied within the scope of the invention
idea.
